Glaucoma is a term which refers to a group of eye diseases that gradually destroy vision. Vision loss due to glaucoma is caused by damage to the optic nerve. High intraocular pressure (IOP) is a risk factor for glaucoma. Glaucoma can also result in high IOP. Glaucoma implants therefore seek to reduce IOP within the eye and to maintain long-term IOP at normal levels.
The glaucoma implant of this invention is a further development and improvement on glaucoma implants invented by the same inventor which have been the subject of earlier patents.
U.S. Pat. No. 4,457,757 describes an implant that is inserted into an eye under the outer tissue layer (Tenon's tissue). The implant comprises a plate which is attached to the surface of the eyeball, and a tube that extends from the plate and is inserted into an anterior chamber of the eye. This enables aqueous fluid to be transferred, via the tube, from the anterior chamber to the plate where the fluid is absorbed into the surrounding tissue. IOP is thereby reduced.
One problem associated with implants of this kind is an excessive lowering of IOP following implantation (post-operative hypotony). This can cause damage to the eye resulting in impairment of vision.
U.S. Pat. No. 4,750,901 describes an improved implant that seeks to prevent post-operative hypotony. This implant has two elevated ridges on the upper surface of the plate. One ridge runs around the edge, or near to the edge, of the plate to help define and maintain a bleb into which fluid from the eye can drain and be absorbed into the overlaying Tenon's tissue. A subsidiary ridge defines a second smaller region within the area of the plate defined by the peripheral edge. During implantation, Tenon's tissue is drawn forward and stretched to cover the subsidiary ridge of the plate of the implant and sutured firmly to the sclera just in front of the plate. This creates a chamber into which fluid from the anterior chamber of the eye is drained. The tension that the tissue exerts on the subsidiary ridge enables fluid to escape on to the plate beyond the subsidiary ridge only when the pressure reaches a certain level. This reduces the possibility of fluid from the eye being too readily absorbed into the Tenon's tissue overlying the plate in the initial few days after surgery and leading to undesirable excessive IOP reduction.
The plate of the implant described in U.S. Pat. No. 4,750,901 may be linked to one or more other plates with interconnecting tubes to allow passage of fluid from the first (primary) plate to subsidiary plates. This provides a greater surface area for more efficient dispersion of large amounts of aqueous fluid. However, the implantation of more than one plate and tube arrangement greatly increases the complexity and difficulty of surgical implantation. The subsidiary ridge principle described in U.S. Pat. No. 4,750,901 can be applied to any subsidiary plate.
Other glaucoma implants are also known, but these all have problems or disadvantages.
U.S. Pay. No. 5,178,604 (Baerveldt) describes an implant made from a soft elastomeric material for the purpose of avoiding trauma to the eye that can occur with an implant made from a rigid material. However, no such trauma or irritation has ever been observed by the inventor for the present application. Baerveldt claims that a softer material is easier to insert into Tenon's capsule. However, U.S. Pat. No. 6,261,256 (Ahmed) indicates that the opposite may in fact be the case.
U.S. Pat. No. 5,300,020 (L'Esperance, Jr.) describes an implant that is a stud designed to keep a channel open for drainage of aqueous fluid from the eye. However, this implant ignores the need for a plate to be attached to the tubular channel to prevent fibrous tissue capping the end of the channel and preventing the escape of fluid.
U.S. Pat. No. 5,370,607 (Memmen) describes a flexible band and reservoir/tube design with baffles on the surface of the reservoir to direct the flow of fluid.
U.S. Pat. No. 5,397,300 (Baerveldt) describes the use of one or more holes in the plate of the implant described in U.S. Pat. No. 5,178,604. The purpose of the holes is to provide tethering by fibrous tissue designed to stop the bleb distending too much and interfering with the action of the eye muscles and causing double vision (diplopia).
U.S. Pat. No. 5,454,796 (Krupin) describes an implant comprising a tube and a valve within the tube. However, as with all valve designs, there is a risk of blockage by blood clot or debris commonly occurring in the fluid of glaucomatous eyes.
U.S. Pat. No. 5,476,445. (Baerveldt) describes a temporary flow restricting seal beneath the plate of an implant. The implant is designed to provide a temporary, non-valved, flow-restricting device aimed at eliminating post-operative hypotony. The walls of the implant form a seal underneath the plate, between the plate and the sclera, into which the fluid drains. The fluid cannot escape until the pressure within the eye builds up beyond what can be sustained by the sutures holding the plate down onto the sclera The sutures are non-permanent and dissolvable. They anchor the plate to the sclera in the immediate post-operative period thereby preventing post-operative hypotony. The sutures then dissolve such that the plate is anchored only by the downward pressure of Tenon's tissue. This reduces the pressure required to effect fluid release from the anterior chamber. However, the placement of these sutures requires considerable additional surgical time. There is also the danger of fibrous tissue in-growth into the cavity nearest the point where the tube enters from the eye.
U.S. Pat. No. 5,558,629 (Baerveldt) describes an implant similar to the implant of U.S. Pat. No. 5,178,604, as well as additional features including a radio-opaque plate, the use of an absorbable ligature around the tube of the implant, the use of an absorbable plug to restrict the escape of aqueous fluid in the immediate post-operative period, the use of double lumen tubing, and the use of two parallel tubes. This implant would suffer from the same problems experienced by the implant of U.S. Pat. No. 5,476,445 discussed above.
U.S. Pat. No. 5,616,118 (Ahmed) describes an implant having a membrane valve system. The valve opens and closes depending on the intraocular pressure. Like all valved systems it carries the risk of blockage by blood colt or fibrin which would necessitate further surgery or removal of the implant. U.S. Pat. No. 5,681,275 (Ahmed) describes a double plate version and U.S. Pat. No. 5,785,674 (Ahmed) describes a winged version intended to stop strabismus (squinting).
U.S. Pat. No. 6,261,256 (Ahmed) describes an implant having ridges on the surface of the plate to stop it rolling up during insertion. However, in practise, creating multiple blebs by holes in the ridges to provide anchorage for fibrous tissue will tend to reduce the effectiveness of the device and result in higher long term intraocular pressures.
U.S. Pat. No. 6,468,283 (Richter) describes an implant of the stud design. The implant has a small disk at the outer end with a ridge intended to prevent blockage by fibrous tissue. In practise this disk is too small in the vast majority of cases and a cap of fibrous tissue will form over the disk preventing the drainage of fluid.
There is, therefore, a need for further improved implants. In particular, there is demand for a greater drainage area on a single plate to allow for efficient aqueous dispersion while minimising the complexity of surgical implantation. In addition, a lower profile implant is desirable to further assist insertion of the implant into Tenon's capsule. There is also a demand for improved contact between the subsidiary ridge and the overlying Tenon's tissue in order to raise the pressure at which fluid lifts the tissue off the subsidiary ridge and drains onto the larger area of the plate. This would improve the ability of the implant to prevent post-operative hypotony.
It is therefore an object of this invention to provide an implant for alleviating problems associated with glaucoma which avoids to at least some extent the problems associated with known implants, or to at least provide a useful alternative to known implants.